The venom of Onyx Cobras can trigger Gembond in dragons.
It is typically more difficult for a dragon with Gembond to fly; however, it also provides additional protection, making it more difficult to penetrate their hide in combat. They may not be able to fly as well but that doesn't mean they are easy targets.
Gembond was the only tertiary gene that could be bought with treasure in the early days of the site. Because of this, combined with there being far more primary and secondary genes available for treasure, created a big demand for new tertiary genes.
On certain breeds, Gembond breaks the typical outline of the dragon (like Spines) And may occasionally show from under a skin/accent.